With the benefit of hindsight, Nixon has one of the most remarkable political legacies of any figure in modern American history – winner of four national elections (as president and vice-presidential), on a presidential ballot every year but one from 1952 to 1972, and one of the most dominating political personalities of the second half of the 20th century.

Here was the man who "went to China", spurred détente with the Soviets, signed into law the establishment of the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), took America off the gold standard and ended the Vietnam war.

Of course, on the flip side, he also prolonged the Vietnam war, obstructed justice from the Oval Office, used the Internal Revenue Service (IRS) to go after his political enemies, launched an illegal war in Cambodia, waged a dirty tricks campaign against his opponents, placed Spiro Agnew a heartbeat from the presidency, kept an "enemies list", was recorded in the Oval Office describing Jews as "aggressive, abrasive and obnoxious" and Italians as not having their "heads screwed on tight", ended the Vietnam war with neither peace nor honor, was impeached by Congress, resigned the presidency and left a permanent stain on American democracy … and those are just some of this greatest hits.

Oh, and also, he committed treason.

Now, that is not my description; rather, it reflects the view of President Lyndon B Johnson, who, in the final days of the 1968 presidential election, became convinced that Richard Nixon (who eventually won the race) and his campaign associates were working surreptitiously with the South Vietnamese government to obstruct peace talks between the US and North Vietnam. It is one of the most duplicitous and pernicious moments in Nixon's political career – which, considering his larger crimes, is really saying something.

To provide a bit of context to the charge, it's necessary to step back to the 1968 election. After tumultuous violence and a "police riot" at the Democratic National Convention in Chicago, Nixon entered the fall campaign with a double-digit lead over his Democratic opponent, Hubert Humphrey, and third party candidate George Wallace. Battered over the war in Vietnam and the anti-war sentiment within the party, Humphrey looked like a political dead man walking.

Few at the time believed that Nixon could lose, so insurmountable did his lead appear. But in late September, Vice-President Humphrey broke with Johnson at a speech in Salt Lake City when he called for a conditional bombing halt in Vietnam. At the same time, Humphrey's old union allies launched a massive effort to discredit the surging Wallace and convince their members to pull the lever once again for the Democrats.

Practically overnight, Humphrey's fortunes shifted. His campaign crowds got larger, anti-war hecklers who had bedeviled him for weeks disappeared, fundraising improved and his poll numbers steadily began to improve. With a week to go before election day, an election that had once seemed like a foregone conclusion was now suddenly a contest.

And then came the "October surprise": a potential breakthrough with the North Vietnamese that held the promise of peace talks in Paris and a hope for an end to the war. On 31 October, Johnson announced an eagerly-awaited bombing halt over North Vietnam. The prospects of "peace at hand", many believed, would represent the final piece of the puzzle for the greatest political comeback since Truman in 1948.

The final wrinkle in the negotiations with North Vietnam was the inclusion of the South Vietnamese government at the Paris talks. For obvious reasons, the South Vietnamese leadership was fearful of where such talks might lead – a US withdrawal could spell the end of their government and their nation. But refusing to attend negotiations in Paris would risk the ire of their US allies.

Luckily for them, they had another ally: the Nixon campaign, which was desperately trying to convince South Vietnam's President Thieu to skip the talks and hold out for a better deal if Nixon was to become president.

Enter the most fascinating figure in this tale of intrigue: Anna Chennault, the Chinese-born widow of General Claire Chennault, commander of the legendary Flying Tigers that ran missions in Burma and China during the second world war. She was a woman for whom the cold war was less a geopolitical and ideological struggle, and more a vocation.

Anna Chennault, with Richard Nixon and Henry Kissinger, in 1972

As strident anti-Communist who had long been involved in Republican politics, Chennault served as a back channel for the Nixon campaign to South Vietnam's ambassador to the United States, Bui Diem. Released FBI intercepts show that Chennault was passing messages to Diem urging him to tell the government in Saigon to refuse to attend talks in Paris. Moreover, she was at the same time communicating with Nixon's campaign manager, John Mitchell, who told her he was "speaking on behalf of Mr Nixon. It's very important that our Vietnamese friends understand our Republican position and I hope you have made that clear to them."

It was one of many contacts between the two – contacts that continued even after the 31 October bombing halt speech. In fact, Chennault told Diem to tell Saigon that her "boss" wanted to tell "Diem's boss", to "hold on, we are gonna win." As for who this shadowy "boss" was – well, he had just called her from New Mexico, said Chennault. By sheer coincidence, Spiro Agnew happened to be in Albuquerque that day.

By even less coincidence, Thieu went before a joint session of the South Vietnamese national assembly several days after Johnson's 31 October announcement and said that his government would be not going to Paris, effectively torpedoing the talks and dealing a blow to Humphrey's election chances.

Clark Clifford, who was secretary of defense at the time, offers in his memoirs one of the most authoritative takes on the Chennault incident – and perhaps its most damning indictment:

"What was conveyed to Thieu through the Chennault channel may never fully be known, but there was no doubt that she conveyed a simple and authoritative message from the Nixon camp that was probably decisive in convincing President Thieu to defy President Johnson – thus delaying the negotiations and prolonging the war."

Now, it should be said that there is a real question of whether the South Vietnamese would have participated in talks even without Nixon's intervention; and of course, there's no guarantee that the war would have ended sooner if they had come to Paris. In addition, it's far from clear that Humphrey would have won the 1968 election if Thieu had not refused to attend the talks. But certainly, it is very possible that the war might have ended sooner, and countless lives might have been saved – if Nixon had not muddied the waters with the South Vietnamese.

We, of course, know about the incident now with hindsight. What can definitely be said about it was that Nixon and his associates were integrally involved in an effort to derail a US diplomatic initiative to end the war in Vietnam – and for the most appalling of purposes: to win Richard Nixon a presidential election.

While much of this history has been known for years, it is oddly one of the most forgotten elements of Nixon's odious record in the public spotlight. On some level, though, it is the greatest possible metaphor for Nixon's legacy: that he would without scruple place personal aggrandizement ahead of the national interest.

It's a worthwhile reminder that if one were ever moved to give Richard Nixon the benefit of the doubt, the urge must be resisted.